Canada, Scotland, South Africa, US An underground railway, especially for mass transit of people in urban areas.
He is going to Westminster by subway.
In 1884 Greathead was part of a syndicate that obtained powers for another subway – 'The City of London & Southwark Subway'. The term 'subway' sounded more sophisticated than 'underground railway', which was associated with the sulphurous Metropolitan, and it would be adopted by New York for its own electric metro when work started on that in 1904.2012, Andrew Martin, Underground Overground: A passenger's history of the Tube, Profile Books, →ISBN, page 98:
Just before you leave, the subway comes. You get on. It stops at the next station.1981 April 29, Russel Baker, “And Only Sixty Cents”, in The New York Times:
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Canada, US A rapid transit system, regardless of the elevation of its right of way; a metro system.
4
UK An underground walkway, tunnel for pedestrians (called pedestrian underpass in US).
At Nairobi the mail waits an hour-and-a-half. The station has three long platforms, mostly covered in awnings, the island connected with the main platform (which is used by the mails in both directions) by a subway.1950 April, Timothy H. Cobb, “The Kenya-Uganda Railway”, in Railway Magazine, page 265:
Heading beneath the tracks via the subway to the immediate north of the station takes us to the Didcot Railway Centre.2021 December 29, Stephen Roberts, “Stories and facts behind railway plaques: Didcot (1932)”, in RAIL, number 947, page 60:
I suppose I could have subwayed around town in search of froufrou French pastry shops.2008 February 13, Melissa Clark, “From Paris, With Hustle”, in New York Times: